1. Field of the Invention
The present invention relates to a spin-transfer torque (STT) magnetic random access memory (MRAM), and, more particularly, to an STTMRAM element having magnetic tunnel junctions (MTJs) with ferromagnetic multilayers whose magnetization is oriented perpendicular to the plane of the substrate, and having lower programming current density while maintaining higher thermal stability.
2. Description of the Prior Art
Magnetic random access memory (MRAM) is a type of non-volatile memory in which magnetization of magnetic layers in MTJs switches between parallel (corresponding to a low resistance state) and anti-parallel (corresponding to high resistance state) configurations. One type of MRAM is spin-transfer torque magnetic random access memory (STTMRAM) where switching occurs through the application of spin polarized current across the MTJ during programming.
STTMRAM has significant advantages over magnetic-field-switched (toggle) MRAM, which has been recently commercialized. The main hurdles associated with field-switched-MRAM are its more complex cell architecture with high write current (currently in the order of milliamps (mA)) and poor scalability attributed to the process used to manufacture these devices. That is, these devices cannot scale beyond 65 nanometer (nm) process node. The poor scalability of such devices is intrinsic to the field writing methods. The current generated fields to write the bits increase rapidly as the size of the MTJ elements shrink. STT writing technology allows directly passing a current through the MTJ, thereby overcoming the foregoing hurdles and resulting in much lower switching current [in the order of microamps (uA)], simpler cell architecture which results in a smaller cell size (for single-bit cells) and reduced manufacturing cost, and more importantly, improved scalability.
One of the challenges for implementing STT is a substantial reduction of the intrinsic current density to switch the magnetization of the free layer while maintaining high thermal stability, which is required for long-term data retention. Minimal switching (write) current is required mainly for reducing the size of select transistor of the memory cell, which is typically coupled in series with MTJ, because the channel width of the transistor is proportional to the drive current of the transistor. It is understood that the smaller the STT current, the smaller the transistor size, leading to a smaller memory cell size. A smaller current also leads to smaller voltage across MTJ, which decreases the probability of tunneling barrier degradation and breakdown, ensuring a high write endurance of the MTJ cell. This is particularly important for STTMRAM, because both sense and write currents are driven through MTJ cells.
One of the efficient ways to reduce the programming current in STTMRAM is to use a MTJ with perpendicular anisotropy. Incorporation of conventional perpendicular anisotropy materials, such as FePt, into STTMRAM causes a high damping constant leading to undesirably high switching current density. Furthermore, during manufacturing, conventional higher ordering transformation temperature required for forming L10 order structure could degrade the tunneling magneto-resistance (TMR) performance and make MTJ deposition process more demanding and complicated (such as elevated substrate temperatures during MTJ film deposition).
Prior art techniques rely on intrinsic perpendicular anisotropy of the Fe-rich CoFeB alloys, and on the anisotropy from the interface with the main MgO barrier. Having a single magnetic layer is limiting however. The layer has to be not too thin, otherwise, it will become super-paramagnetic. It cannot be too thick either, otherwise, it will become a layer with in-plane anisotropy. In this small parameter space, one has to make the coercive fields for the pinned and free layers as far apart as possible. Low thermal stability (>40 required) is also an issue and it may be necessary to increase this parameter to 50-60 for a reliable memory product. Thermal stability is defined by KuV/kBT, where Ku represents magnetic anisotropy constant, V represents the volume of the free layer, kB is Boltzmann's constant and T represents temperature.
Thus there is a need to scale the perpendicular anisotropy of the free layer according to its effective magnetic thickness.
There is also a need for a STTMRAM element having a MTJ with perpendicular magnetic anisotropy material(s) with a simple film manufacturing process and an optimal combination of saturation magnetization (Ms) and anisotropy constant (Ku) to lower the damping constant and the magnetic anisotropy of the MTJ yielding a lower STT switching current density while maintaining high thermal stability and high TMR performance.